High-purity germanium (HPGe) detectors (see G. F. Knoll, Radiation Detectors and Measurements, Wiley 1989, Chapters 2, 4, 11 and 12) are frequently used in energy spectroscopy and related fields to detect gamma rays or other high-energy photons. These detectors essentially consist of a large germanium diode made from germanium which is very highly purified, and lightly doped. The germanium crystal is machined in a desired shape, such as planar or coaxial. Electrodes are applied to opposing contacts of the detector, one of which is rectifying electrode formed of p+ or n+ species, depending on the doping of the bulk germanium. A reverse-bias voltage is applied to the electrodes. The voltage is sufficiently high to cause all of the volume of the germanium to be depleted of charge carriers (i.e., under an electric field). Gamma rays impinging upon the detector will collide with the germanium atoms, causing hole-electron carrier pairs to be created. These holes and electrons are collected by the electrodes. The total charge collected by the electrodes is related to the energy of the detected photons.
The energy resolution of these detectors as gamma ray spectrometers is extremely good. For example, for gamma rays of 1 MeV of energy, the energy resolution, measured as the full width half maximum of the gaussian peak generated by the detector-electronics system, is better than two parts per thousand. Also, these detectors are now available in very large size (cylinders of 8 cm diameter by 8 cm length) and have, therefore, a high gamma detection efficiency.
Therefore, notwithstanding the high price and the inconvenience of cooling the detectors at liquid nitrogen temperature, they are the detector of choice in nuclear structure studies. Such studies are nowadays conducted with large arrays ("Gammasphere" at LBL-USA, "Euroball" at Legnato-Italy) where 100 or more detectors are mounted in a spherical structure. Also "Miniballs" (40-60 detectors) are in a project phase. In such experiments, the target, at the center of the sphere, is bombarded with fast heavy ions. The resulting gamma rays provide the information sought by the scientists. However, the nuclei recoil while emitting gammas which causes the emitted gamma energy lines to be broadened by Doppler shift. The only way to mitigate the imprecision caused by the Doppler shift is to correct the spectra by identifying where in the detector the event takes place. This also adds the benefit of complete tracking of the gamma rays inside the detector, thus distinguishing between multiple interactions of a single hit and multiple hits (see Nuclear Instruments and Methods in Physics Research A371 (1996), 489-496).
One modification proposed and tried for obtaining position information is "segmentation", i.e., dividing the outer and/or the inner contact in two or more conducting surfaces electrically insulated from each other by thin separation lines. For example, 60 of the 110 detectors in the Gammasphere are twofold segmented, i.e., the outer contact of the detector is divided in two electrically insulated halves. Signals are obtained both from the central contact (total energy) and the side contacts (position). In order to further improve position resolution, multiple segmentation (for example, the LBL "GRETA" project with 32 segments) is being proposed.
It should be noted that, while segmentation certainly works, it adds a large cost to the system because it is more difficult to make a segmented detector and also, every segmented channel needs a complete line of electronics. Moreover, segmented detectors are inherently less reliable.
It is well known that gamma rays interact with Ge in a complex way, often resulting in multiple interactions.
This would seem to make pulse shape analysis difficult or even impossible. However, in practice, three factors intervene which vastly improve the viability of pulse shape analysis as a position measuring tool. As the energy increases the scattering cross-section becomes more and more peaked in the forward direction. When the scattering angle is large, most of the energy is deposited in the first interaction. At low energies the photoelectric effect (one interaction) prevails. Because of these factors it appears that, notwithstanding the existence of multiple scattering, such scattering decreases the precision of the measurement, but does not substantially impair the viability of signal pulse shape analysis as a position-measuring tool.
High purity germanium detectors are used for precision measurements of gamma-ray energy. In many applications it is also useful to know the point or points of interaction of the gamma-ray photon in the germanium detector. This is often accomplished by separating the electrodes into separate segments and noting which segments collect charge from the photon interaction.
The patent application Asymmetric Radiation Detector System, by Martini, Gedcke, Raudorf and Sangsingkeow, filed Mar. 26, 1997, Ser. No. 08/824,514, of which this is a continuation-in-part, describes a method of obtaining azimuthal and radial position information without such segmentation or with only a minimal amount of segmentation. Martini et al. show that by introducing a degree of asymmetry to the electrodes and analyzing the rise time of the detector signal, position information can be obtained in a plane generally perpendicular to the axis of the detector.
If the photon has energy above a few hundred keV, it will usually interact with the germanium crystal at several points before losing all its energy. The interactions are physically well separated in the crystal, with an average separation distance of about one centimeter. The interactions occur at essentially one point in time, since the photon is moving at the speed of light and the detector dimensions are relatively small. The method of Martini et al. measures the energy-weighted average azimuthal and radial position of the interactions.
There are cases in which it is useful to know the total number of interactions and their position in the detector. For example, in the "crystal ball" detector systems used in physics research, the scientists would like to track each photon interaction through the detector system. Such tracking requires each detector to give position information in all three spatial dimensions. It is known that position information along the axis of the detector can be obtained by dividing the outer electrode into segments perpendicular to the detector axis, i.e., along the longitudinal axis. This method is discussed, for example, in the paper by Varnell et al., IEEE Trans. Nucl. Sci. 31, 300 (1984).